The present invention relates to a catalytic promoter used in the process of fluid catalytic cracking of hydrocarbons in petroleum refining field. More particularly, it relates to a catalytic promoter used in the process of fluid catalytic cracking of hydrocarbons to reduce olefin content in gasoline from catalytic cracking process, and to the preparation thereof.
Gasoline octane number is one of the most important indexes to evaluate motor gasoline. The operating conditions comprising short contact time at an elevated temperature in the presence of an ultrastable zeolite catalyst are now generally employed in a unit for catalytic cracking in a refinery in order to increase gasoline octane number. Such a technique, however, results in increased olefins in gasoline from catalytic cracking process. Olefins in gasoline constitute the major reason for the high amount of contaminants from automobile exhaust emissions. It is necessary to reduce the olefin content in gasoline to meet environmental requirements. Thus, there""s a great need to reduce olefin content in catalytically cracked productsxe2x80x94gasoline, and meanwhile to increase or maintain gasoline octane number.
The use of catalytic cracking catalysts containing high silica zeolite, ZSM-5, to increase gasoline octane number, is well known to one skilled in the art. As disclosed in the art, a ZSM-5 zeolite can be incorporated into a catalytic cracking catalyst as one of the active components. Alternatively, the ZSM-5 zeolite as a single active component can be made into an additive catalyst which is then added to a unit for catalytic cracking in combination with a conventional catalytic cracking catalyst. The amount of the ZSM-5 zeolite added can vary widely; it can be either as little as 0.01-1 wt % or as much as 5-10 wt % or even more, based on the total weight of the catalyst. As disclosed in U.S. Pat. No. 4,929,337, a ZSM-5 zeolite modified with Ga is used as an active component for aromatization. Alkanes and olefins are subjected to cracking/aromatization by means of the shape selective cracking/aromatization component. The shape selective aromatization component converts alkanes and olefins to aromatic hydrocarbons. These catalysts, however, are mainly used in catalytic cracking to increase gasoline octane number, without an effect of reducing olefin content in gasoline. A process of catalytic cracking of heavy oils to increase gasoline octane number is disclosed in U.S. Pat. No. 4,867,863. A catalyst containing 0.5-5% HZSM-5 zeolite was used in the patent, with the result of an increased gasoline octane number. But the effect of reducing olefin content in gasoline was not indicated.
It is an object of the present invention to provide a catalytic promoter, which can not only reduce olefin content in gasoline but also increase gasoline octane number, at the same time, increase the yield of very useful industrial feedstock, lower olefins. It is another object of the present invention to provide a method for preparing said catalytic promoter.
The present catalytic promoter for the catalytic cracking of hydrocarbons comprises a HZSM-5 zeolite in an amount of 5-65%, and preferably 20-50% based on weight of the catalytic promoter. The HZSM-5 zeolite is modified with zinc and at least one element selected from a group consisting of P, Ga, Al, Ni and rare earth elements or their mixtures. The amount of the modifying elements in HZSM-5 can be adjusted depending on the catalytic cracking feedstock, the main catalytic cracking catalyst and the desired catalytic cracking effect, it being preferably 0.01-10.37 wt %, and more preferably 0.1-5.0 wt %. The applicant also discovered that the most desirable results are achieved by utilizing the present catalytic promoter containing 23%-95.3% by weight of zinc based on the total weight of the modifying elements.
The HZSM-5 zeolite used in this invention can be any of the commercially available products. The HZSM-5 zeolite with a SiO2/Al2O3 ratio of preferably 20-500, more preferably 30-200, and most preferably 30-100, is highly preferred.
ZSM type series zeolites whose representative is HZSM-5 are such a kind of zeolite that have high silica content and structure of three-dimensional crossed straight channel. They are characterized by lipophilicity, hydrophobicity and good thermostability etc. The majority of their pores are about 5.5 xc3x85 in size. The catalytic activity of ZSM type relies on the surface acidity resulting from acid sites therein. Most of acid sites are located in the cavities of zeolites. ZSM type zeolites exhibit specific shape-selective catalyzing as a result of the particular pore path structure in conjunction with acid sites. Different acid-catalyzed reactions require different strength of acidic sites. The strength of acidic sites has a direct effect on the selectivity of a reaction. It is necessary to suitably adjust the amount, strength and distribution of acidic sites in the zeolites in order to induce a desirable reaction. For such a purpose, to modify zeolites is one of the most effective methods. In present invention some elements are incorporated into zeolites to effect such modification.
There are a wide variety of methods for incorporating some elements into zeolites, such as ion-exchange, impregnation and high temperature processing as disclosed in CN1,057,408A. The location where the incorporated element is in the zeolite, varies with the method employed, and has an influence on the properties of the zeolite. A modified zeolite is prepared by the impregnation method in present invention. The method of impregnation is simple and readily carried out. The impregnated zeolite or catalyst is dried and calcined, and then the modifying element is deposited on the zeolite and bound thereto, acting to adjust the acidity of the zeolite. The typical method for a zeolite modification in present invention comprises impregnating a HZSM-5 zeolite with an aqueous solution containing a compound of a modifying element and then drying and calcining the impregnated zeolite. Said compound can be any of water soluble compounds such as halide and inorganic or organic acid salts of the modifying element, as long as the modifying element therein can deposit on the zeolite and bind thereto during impregnation. Chloride, nitrate and sulfate salts are preferred. The zeolite can be impregnated with different solutions containing compounds of different modifying elements simultaneously or separately. The concentration of the modifying element in an aqueous solution is determined by the content of modifying elements required in said zeolite. The zeolite is normally impregnated with the above aqueous solution at 10-60xc2x0 C. for 1-30 hours. The impregnated zeolite is usually dried at 100-160xc2x0 C. for 1-8 hours and then calcined at an elevated temperature of 500-600xc2x0 C. for 1-8 hours prior to pulverization. Fine powders of particle size greater than 150 mesh American Standard Screen are collected.
When modification of said zeolite is completed as described above, the present catalytic promoter is subject to a forming process to produce the product of final use. An effective amount of supports and binders also need to be added during forming. Preferably a support in an amount of 15-60% and a binder in an amount of 10-40% based on the total weight of the formed catalytic promoter product are added. It is preferred to use at least one material selected from ZrO2, P2O5, clay, diatomaceous earth or sepiolite or mixtures thereof as the support. At least one material selected from alumina sol, silica-alumina sol, aluminum phosphate sol or mixtures thereof is preferably used as the binder.
The catalytic promoters are formed as follows. Said modified HZSM-5 zeolite is added into an effective amount of binders together with an effective amount of the support to produce a mixture. The obtained mixture is then subject to a slurrying and homogenizing step, while the pH is controlled at 2.5-4.8. The homogenized mixture is sprayed drying, washed with water, and then dried for about 4 hours to obtain the catalytic promoter products.
The binder used can be the commercially available product. There are various methods for preparing said binder. For example, the alumina sol binder can be obtained by dissolving the aluminum metal in hydrochloric acid aqueous solution or by dissolving the pseudoboehmite in nitric acid or hydrochloric acid aqueous solution while heating. The alumina is present, in the sol of pH 2.0-3.5, in the range of 10-30wt %. See Examples 12-14 in CN1072201A for the preparation of silica-alumina sol binder. To be specific, 567 g of 25% sulfuric acid solution is added to 120 g of purified water. Then 0.19 L of aluminum sulfate aqueous solution containing 90 g/L Al2O3 is added. The mixture is stirred well and cooled to below 10xc2x0 C. 2.36 L of water glass containing 124 g/l SiO2 (modulus 3.2-3.3) is added while stirring to obtain the silica-alumina sol. Aluminum phosphate sol can be made by neutralization method. Namely, an appropriate amount of water is added to 85% phosphoric acid. The solution is heated to about 110xc2x0 C. Aluminum hydroxide solid powder (96% purity) is added slowly. The temperature is maintained above 100xc2x0 C. for one hour. The pH is adjusted to be about 1.4 to obtain the aluminum phosphate sol, which contains about 35% P2O5 and about 8.5% Al2O3.
The present catalytic promoters for the catalytic cracking of hydrocarbons are suitable to process of fluid catalytic cracking of hydrocarbons, in particular to the catalytic cracking of vacuum gas oils (VGO), atmospheric resid or VGO blended with vacuum resid. The present catalytic promoters can be employed in combination of various conventional catalysts for the fluid catalytic cracking of hydrocarbons. The amount of the present catalytic promoters added can vary with the feedstock to be catalytically cracked, the main catalyst used and the desired catalytic cracking effect, generally it being 1-15% based on the catalyst inventory in unit, i.e. the combined weight of main catalyst and the catalytic promoters charged into the catalytic cracking unit. The present catalytic promoter product is used in the form of finely-divided powders, the bulk density of which being less than 1.0 g/ml and the particle size distribution being as follows, thus producing a catalyst maintaining the fluid state during the reaction and regeneration process:
0-40 xcexcm, not more than 20 wt %
40-80 xcexcm, not less than 55 wt %
 greater than 80 xcexcm, not more than 25 wt %
The present catalytic promoter can be blended with the main catalyst for catalytic cracking and then added to the reaction system through a feeding system connecting a catalyst storage tank. Alternatively, the present catalytic promoter can be added to the reaction system separately through a small catalyst feeding system.
The addition of a small amount of the present catalytic promoter to a catalytic cracking unit under conventional catalytic cracking conditions can promote the cracking, hydrogen transfer and aromatization of olefins in gasoline fraction as well as isomerization of hydrocarbons. Thus the advantages of the present invention compared with prior art lie in a reduced olefin content in gasoline from catalytic cracking, an increased gasoline octane number and an increase in the yield of lower olefins.
The present catalytic promoter meets the use requirement of a conventional catalytic cracking process. Heavy oils could be employed as feedstock to produce high-quality gasoline and diesel oil together with a significant amount of liquefied petroleum gas under conventional catalytic cracking conditions. For example, said reaction is carried out at 480-540xc2x0 C., preferably at 490-535xc2x0 C., and more preferably at 500-530xc2x0 C., for 0.5-20 seconds, preferably 0.75-15 seconds, and more preferably 1-7 seconds. Therefore, both reduced olefin content in gasoline and an increased gasoline octane number can be obtained from heavy oils such as atmospheric resid, vacuum resid or mixtures thereof. Accordingly, the quality of gasoline is improved, together with an increase in the yield of liquefied petroleum gas.
The advantages of the present invention will become more apparent with reference to following examples, but these examples are not construed with limiting the present invention. The protection scope of the present invention is defined by the appended claims.
All percentages or ratios used are by weight unless otherwise stated.
Examples 1-16 illustrate the preparation of the modified zeolite in this invention. Examples 17-22 illustrate the preparation of formed catalytic promoter product in this invention. The performance experiments of the present catalytic promoter products are illustrated in Example 23.